This project will investigate the neural and endocrine mechanisms that control social behavior in Syrian hamsters (Mesocricetus auratus). There is a substantial body of evidence that the actions of arginine-vasopressin (AVP) within a zone that extends from the medial preoptic area to the anterior hypothalamus (referred to as the MPOA-AH) plays a critical role in the expression of a form of scent marking called flank marking as well as aggression. Understanding the neurobiological mechanisms controlling social behaviors like flank marking and aggression requires definition of the neural/hormonal mechanisms that control these behaviors as well as how experience changes these mechanisms so that these behaviors are continuously adjusted to the changing environment. The proposed experiments will test the working hypothesis that experience alters the expression of flank marking and aggression by its effects on AVP receptors within the MPOA-AH. This hypothesis will be tested by examining the effects of two types of experience that have significant, yet very different effects on scent marking and aggression. First, we will determine whether social experience (i.e. prior agonistic encounters) alters these receptors and will test the prediction that a testosterone-dependent as well as a testosterone-independent mechanism mediates these effects. Second, we will determine whether environmental experience (i.e. exposure to winter-like short photoperiods) alters these receptors and will test the prediction that a testosterone-independent mechanism mediates these effects. We believe that analysis of the effects of such different types of experience on these AVP receptors will provide a rigorous initial test of our hypothesis that these receptors represent a critical regulatory site in the control of scent marking and aggression. These data should provide new insights into the basic principles that govern how neural and endocrine mechanisms control complex social behaviors. This information is critically important for understanding the etiology of a variety of behavioral and emotional disorders as well as the development of new treatments for these disorders.